This invention is generally concerned with the purification of liquids. Of particular interest is the treatment of water which contains dissolved salts, such as seawater, brackish water or hard water. Such waters may be purified by forcing the water through a semipermeable reverse osmosis membrane, leaving behind the contaminants or salts which do not pass through the membrane.
A reverse osmosis membrane typically rejects a high fraction of the dissolved salts. It is also important that such membranes pass a relatively large amount of water (i.e., have a high water flux) through the membrane at relatively low pressures. In addition, the membrane must be tolerant to chlorine, if added as a disinfectant. Although pretreatment to remove the chlorine is often employed to protect the membrane, the treatment may not always be effective and some chlorine tolerance is desirable.
Many U.S. patents describe membranes which are useful in desalination processes, see for example, those cited and discussed in U.S. Pat. No. 4,830,885 to Tran et al. One of the earliest patents to describe membranes of the type used in the present invention is U.S. Pat. No. 3,744,642 to Scala et al.
The semipermeable membrane used in the desalination process ordinarily will be thin in order to maximize the water flux. Thus the membrane often is formed on a porous support to provide strength, the combination being referred to as a thin film composite (TFC) membrane. The supports should have pores which are sufficiently large so that the water (permeate) can pass through the support without reducing the flux of the entire composite. Conversely, the pores should not be so large that the thin semipermeable membrane will be unable to bridge the pores or will fill up or penetrate too far into the pores. Scala et al. suggest that with pores above about 8 microns (8000 nm) the rejection of impurities is reduced.
In general, those skilled in the art have preferred to use primary amines to form thin film composite membranes by reaction with aromatic polyacyl halides. This is despite the belief that chlorine will attack the hydrogen in the amide group --CO--NH--, as suggested in U.S. Pat. No. 3,951,815. Based on such reasoning, secondary amines have been suggested in U.S. Pat. No. 4,259,183, which employed aliphatic or cycloaliphatic secondary amines, preferably piperazine.
The patentee of the '183 patent also claimed in U.S. Pat. No. 4,277,344 that primary amine groups should be used with aromatic substrates rather than secondary amine groups and noted that adequate chlorine resistance was obtained despite the presence of amidic hydrogen (i.e. from a primary amine).
We have now found that an aromatic polyamine which contains secondary amine groups can be used to prepare thin film composite membranes which can provide adequate chlorine tolerance, high water flux and salt rejection. Such membranes have been found to have more stable water flux than those made with primary amine groups, especially for waters containing chemically active organic solutes.